The present invention relates to a method and apparatus for controlling the sub-surface temperature of a grass field which is subjected to less than optimal growing conditions. The method and apparatus have application, for example, in dramatically improving the growing conditions of golfing greens which are subject to shading from sunlight and temperature extremes which are harmful to grass surfaces.
It is well known in the prior art pertaining generally to plant growing that such growth is largely dependent on such factors as the extent to which a plant is exposed to sunlight and the temperature to which the root system is exposed.
Regarding temperature, it is well known that plant growth can be facilitated by maintaining plant growing temperatures within an optimal range. Systems for controlling the ambient temperature of plants have primarily been directed to the use of enclosures, such as greenhouses, in which the overall ambient air temperature is controlled, usually by varying radiant heat from solar or artificial sources, use of heat pipes in contact with the air, or air heat pumps or air conditioners, or various combinations of such methods.
Due to the substantial cost of energy required to control ambient temperature, energy efficiency in connection with the growing of plants has become a significant consideration. It is now widely recognized in the industry that one of the most efficient ways of growing plants is to create a micro-climate for the plants, through heating of the root zones of the plants, rather than by attempting to control the air temperature around the plants. In some installations it has been even found most advantageous to heat the plant root zone while cooling the foliage. Therefore, considerable effort has been expended in developing water-based heating systems which are installed underneath plant containers or in propagation benches or beds in combination with space heating or cooling apparatus. Such root heating or micro-climate temperature control systems have been found to achieve substantial energy savings as compared to conventional space heating systems. Unfortunately, such heating systems have also been accompanied by a significant increase in the system maintenance costs.
A patent disclosing such a system is U.S. Pat. No. 4,557,435 to Springer at al. This patent discloses a micro-climate temperature control apparatus wherein an array of heat-exchanging plastic tubes are disposed adjacent a plurality of plants in individual growing containers. Each plastic tube is connected to an inlet and outlet manifold which in turn are connected to a source of heated water.
U.S. Pat. No. 4,309,843 to Kato discloses a plant cultivation device using hot water pipes covered by a layer of wood chips and a second layer of soil. The heating pipes are enclosed in a complex structure including concrete blocks, slate-corrugated plates, partition wall plates and a shielding cover. Maintenance of such a system poses significant problems due to the inability to access manifolds in the event of leakage which can lead to destruction of root systems.
U.S. Pat. No. 5,120,158 to Husu discloses an apparatus for thawing a frozen grass playing field wherein an array of perforated pipes are arranged in a filter layer beneath the playing field. The perforated pipes are connected to blowers which circulate heated air through the pipes in order to thaw the frozen field and remove moisture therefrom.
Although such a system may be appropriate for raising the temperature of a field above the freezing point, and thereby thawing the field, temperature gradients which exist in a volume of air significantly limit the utility of such a system to control temperatures of plants in contact with such air. Further, such convective methods require considerably more time than a conductive heat transfer method to bring the overall temperature of a growing system to steady state conditions or to correct variations which occur due to factors outside the enclosed growing environment. Moreover, the heating and cooling of large volumes of air is generally less efficient and more costly as a heat transfer method than controlling a confined and smaller volume of water or other liquid exhibiting a greater heat conductivity than air.
Subsurface hydronic heating systems have been installed in golfing greens to control the green temperature and promote growth. However, significant drawbacks are associated with prior art systems which limit their overall utility. For example, such systems generally entail installation of a subsurface header assembly within the green area, thereby requiring major excavation of the green in the event of leakage at the headers or for completion of maintenance procedures. Also, prior art designs necessitate the use of excessive lengths of subsurface tubing which may result in uneven heat transfer and uneven heating of the green. Moreover, temperature sensor and control systems responsive to localized temperature fluctuations frequently result in the over heating or over cooling of regions of the green not subject to the localized temperature fluctuation, thereby resulting in less than optimum control of the temperature of the turf field and in a waste of energy.
Accordingly, one object of the present invention is the provision of an apparatus and method for maintaining the temperature of a grass field within a specified range, thereby promoting optimum growth of the field. Another object of the present invention is the provision of an apparatus and method for controlling the temperature of a turf field which permits maintenance and repair of a subsurface header assembly to be conducted in a simple and convenient manner without destructive excavation of the grass field. A further objective of the present invention is the provision of an apparatus and method which permits even heating throughout the grass field. An even further objective of the present invention is the provision of a turf heating apparatus and method which is leakage resistant and significantly more energy efficient than prior art devices and methods.
These and other objects of the present invention are attained by the provision of subsurface hydronic heat-exchanging apparatus and method wherein a temperature-controlling fluid is circulated from a temperature-controlled fluid source through a supply header to an array of hollow tubular heat-exchanging elements and back through a return header to the source. The headers are connected to the heat-exchanging elements through tubular extensions which permit placement of the header at a position outside the turf field, thereby permitting access to the subsurface headers without destructive excavation of the field. Temperature sensor means are disposed adjacent the turf field which detect the temperature of the field and temperature control means are provided which are responsive to the detected temperature.
In one preferred embodiment, multiple sensors are disposed at selected points throughout the field which transmit temperature information to a device which computes an average field temperature and a control device is provided which controls the temperature or flow rate of the temperature controlling fluid in response to the average field temperature, thereby minimizing the effect of localized temperature extremes in the field.
In another preferred embodiment, specially designed wedge-like coupling devices are employed to fluidly connect the large diameter tubular extensions to the supply and return headers with a leak resistant interference fit. The special couplings provide critical leak resistance while permitting the use of large diameter heat-exchanging tubes which reduces installation time, simplify plumbing and permit the use of complete circuits of substantially increased length without compromising heating or cooling evenness.
Other objects and advantages of the present invention will be more apparent from the following detailed description of preferred embodiments, when taken in conjunction with the drawings.